The head of France’s national police force on Tuesday ordered an internal investigation after hundreds of police officers held an unauthorized protest in central Paris overnight.

Angry police officers marched on the iconic Champs Elysées boulevard in the French capital after dark on Monday, complaining that they are understaffed and ill-equipped.

The rebel police officers, who held the demonstration without permission or backing of their labour unions, accused Prime Minister Manuel Valls and Interior Minister Bernard Cazeneuve of grandstanding while failing to provide the resources they need to do their jobs.

They also protested rising violence against police officers and forced overtime, amid more than 18 months of Islamist militant attacks that have left France in a state of heightened security.

While strikes and street protests are common in France, police officers are among a small minority of state workers who are banned from participating in such events. Monday night’s police protest was reportedly a spontaneous march, with plain clothes officers in some cases using squad cars.

They sang the Marseillaise national anthem as they rounded the Arc de Triomphe monument.

Jean-Marc Falcone, director of France’s national police force, condemned the “inacceptable behaviour” of a handful of officers, announcing he was opening a public investigation into the matter.

“These public demonstrations by policemen go against their legal work contracts. This behaviour endangers the national police and endangers each police officer,” Falcone wrote in a statement.

A few right-wing and far-right politicians voiced their support for the protesting policemen.

The government of Socialist President François Hollande, who faces presidential and legislative elections in the first quarter of 2017, has started restoring thousands of police posts axed by former president Nicolas Sarkozy.